This sounds silly but anyway.
On golfs and ibizas usually you have the coolant pipe going in at the front of the engine, if your car is flat or sloped down hill you can have an issue with the coolant only filling the radiator and pump side, and not getting to the engine.
I had this
problem and the car would start and then stop. I solved it by jacking it up. which meant the coolant could flow in to the engine side, and the air would more or less to where the pipe to the expansion reservior goes.